This section introduces aspects that may facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
It is envisioned that wireless data traffic will become more and more localized in the future, where most users will be in hotspots, or in indoor areas, or in residential areas. These users will be located in clusters and will produce different uplink (UL) and downlink (DL) traffic at different time. This essentially means that a dynamic feature to adjust the UL and DL resources to instantaneous (or short term) traffic variations would be required in future local area cells. In this case, a Time Division Duplex (TDD) system which has the flexibility to dynamically allocate the UL/DL resources becomes very attractive.
There are seven different TDD UL/DL configurations in Long-Term Evolution (LTE), providing about 40%-90% resources for DL. Faster TDD reconfigurations (referred to “dynamic TDD” henceforth) have shown good performance potentials in both UL and DL especially at low to medium system load and will be a standardized feature of LTE Release 12.
Different signaling methods are being considered to support dynamic TDD reconfigurations with different time scale. In the 3rd Generation Partnership Project (3GPP) RN1#73 Chairman's Notes, May 20-24, 2013, an explicit L1 signaling is proposed to support dynamic TDD. With L1 signaling, the link direction of the flexible sub-frame is controlled by the enhanced-NodeB (eNB) and the user equipment (UE) will follow the signaling to judge whether the sub-frame is a downlink or an uplink.
In 3GPP Release 10, carrier aggregation (CA) was introduced to support even higher data rates. LTE-Advanced (LTE-A) aims to support peak data rates of 1 Gbps in the downlink and 500 Mbps in the uplink. In order to fulfill such requirements, a transmission bandwidth of up to 100 MHz is required; however, since the availability of such large portions of contiguous spectrum is rare in practice, LTE-A uses carrier aggregation of multiple Component Carriers (CCs) to achieve high-bandwidth transmission. Release 8 LTE carriers have a maximum bandwidth of 20 MHz, so LTE-A supports aggregation of up to five 20 MHz CCs.
For backward compatibility, each CC appears as a separate cell with its own Cell ID there is one primary CC (PCC, or referred to as primary cell, PCell) configured for each UE, including the primary UL carrier and primary DL carrier. Other carriers configured for the UE are referred as secondary CCs (SCCs, or referred to as secondary cells, SCells). The PCell is defined as the cell that is initially configured during connection establishment; it plays an essential role with respect to security, Non-Access Stratum 2 (NAS2) mobility information, System Information (SI) for configured cells (i.e. carriers), and some lower-layer functions. A SCell is a cell that may be configured after connection establishment, merely to provide additional radio resources.
There are two types of scheduling policies defined:
Backward compatible method (i.e. non-cross-carrier scheduling or self scheduling): as in Release 8, it is possible for a physical dedicated control channel (PDCCH) on each downlink CC to carry downlink resource assignments applicable to the same CC, and uplink resource grants applicable to the associated uplink CC (according to the linkage indicated in System Information Block 2 (SIB2)).
Cross-carrier scheduling method: this enables a PDCCH on one CC to schedule data transmissions on another CC by means of a new 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the PDCCH messages. The rest of the Release 8 PDCCH Control Channel Element (CCE) structure, coding and message are unchanged for carrier aggregation. The presence or absence of the CIF on each CC is configured semi-statically (i.e. by RRC signaling) for each UE. When configured, the CIF is only present in PDCCH messages in the UE-specific search space not the common search space. The UE listens to the downlink assignment and uplink grant over the configured CCs.
Different UEs may be configured with different PCC and SCCs. The configured carriers for downlink assignment and uplink grant transmission can be different.
Considering the high data rate requirement in future, it is naturally that carrier aggregation will be implemented in micro and pico nodes. In case of carrier aggregation, one UE may be served by multiple carriers and different UEs may be served by different carriers. Different UEs may be configured with different Primary Component Carriers (PCC) and Secondary Component Carriers (SCC). Further, different UEs may be configured to monitor the downlink assignments and uplink grants over different carriers. Currently dynamic TDD is being introduced into 3GPP and the claimed main applicable scenario of dynamic TDD is the micro/pico nodes. Thus it is meaningful to propose methods to indicate the UL-DL configuration switch in case of carrier aggregation case.
In 3GPP, explicit physical layer signaling is proposed to be the work assumption, while the signal in case of carrier aggregation case is not mentioned yet. Hence it is meaningful to propose some methods to notify UEs regarding the UL-DL configuration change.